Gas turbines are widely used in industrial and power generation operations. A gas turbine generally includes, in serial flow order, a compressor, a combustion section and a turbine. The combustion section may include multiple combustors annularly arranged around an outer casing. In operation, a working fluid such as ambient air is progressively compressed as it flows through the compressor. A portion of the compressed working fluid is routed from the compressor to each of the combustors where it is mixed with a fuel and burned in a combustion zone to produce combustion gases. The combustion gases are routed through the turbine along a hot gas path where thermal and/or kinetic energy is extracted from the combustion gases via turbine rotors blades coupled to a rotor shaft, thus causing the rotor shaft to rotate and produce work and/or thrust.
Some combustion systems utilize a plurality of premix type fuel nozzles. For example, some combustors include a center or primary premix fuel nozzle and a plurality of secondary premix fuel nozzles annularly arranged around the center fuel nozzle. This arrangement of fuel nozzles may provide for fuel staging, desired emissions performance, and flame stability.
At least one of the fuel nozzles may include a premix pilot nozzle. The premix pilot nozzle may be coaxially aligned with a center body portion of the corresponding fuel nozzle and may be disposed at a distal end of the center body upstream from the combustion zone. During particular combustion operation modes, the premix pilot nozzle may deliver a premixed fuel and air mixture to the combustion zone to produce a pilot flame. The pilot flame is generally used to ensure flame stability as the combustor is operated in certain modes and/or when the combustor transitions between various modes of operation.
The premix pilot nozzle generally includes a tip portion having a flat or planer downstream surface that is positioned proximate to the combustion zone. Multiple fuel ports and/or air passages extend through the downstream surface and provide for fluid communication of the premixed fuel and air out of the tip portion. The base of the pilot flame resides adjacent to or just downstream from the downstream surface. As a result, the downstream surface is exposed to extremely high temperatures.
One solution for cooling the downstream surface of the tip portion may include directing air across an upstream or backside or surface of the tip. Another technique for cooling the downstream surface may include directing cooling air across the generally planer downstream surface. However, this technique may result in flame instability when the cooling air strikes the pilot flame at or near the base of the pilot flame. In addition or in the alternative, various coatings such as thermal barrier coatings and/or anti-oxidation coatings may be applied to the downstream surface to achieve desired component life, reduce thermal stresses and to reduce deposit formation on the downstream surface.
Although these solutions are effective for reducing or managing cooling of the tip portion of a pilot premix nozzle, an improved premix pilot nozzle that reduces flame instability while providing cooling to the downstream end of the tip portion would be useful in the art.